The present invention relates to an optical information recording medium and methods for recording, reading, and erasing information using laser light and, more particularly, to an optical information recording medium adapted to recording of information at a high density and methods of recording, reading, and erasing information on such an optical information recording medium.
Optical disk recording using laser light is being put into practical use as large-capacity memories because large amounts of information can be recorded and non-contact, high-speed access is permitted. Optical disks are classified into (i) read-only type, such as compact disks and laser disks, (ii) write-once-read-many (WORM) on which the user himself or herself can record, and (iii) rewritable type permitting the user to record and erase repeatedly. WORM and rewritable optical disks are used as external memories for computers and as recording media for storing text and image files.
With respect to a read-only optical disk, those adapted for data files typified by CD-ROMs have rapidly become widespread and are used as high-density recording media in personal applications. In view of this large capacity of CD-ROMs, it has been discussed that read-only optical disks will be used to hold multimedia files including image data by making use of image compression techniques such as MPEG-2. In these applications, the existing capacity of 650 MB per disk is not sufficient. Therefore, DVD-ROMs having a capacity 8 times as large as that of the present media have started to be put on the market. On the other hand, with respect to read-only products used in civil applications for image files as typified by laser disks, DVD players providing small-sized high-quality images have started to appear on the market. However, higher-density recording on optical disks aimed at high-definition reproduction remains an important issue.
On the other hand, the write-once-read-many (WORM) optical disk has the advantage that recorded information can be stored stably. In applications where full use is made of this advantage, WORM optical disks have a certain share of the market. In addition, in these applications, large capacities and higher densities are important in making use of the merit of large scale.
Rewritable optical disks include phase-change optical disks utilizing phase change of recording layers and magneto-optical (M-O) disks employing changes in vertical magnetization film in the direction of magnetization. Among them, MO disks having 4 times capacity of first generation has started to appear on the market. Disks having 8 times or more capacities are being discussed. On the other hand, phase-change optical disks need no external magnetic field and can easily implement overwrite. It is expected that phase-change optical disks will become the mainstream of rewritable optical disks in future, together with MO disks. Also in the case of phase-change optical disks, commercial products having 8 times the capacity of that of the MO disk have started to appear on the market, as typified by 2.6 GB DVD-RAMs. Larger capacities and higher densities are also important for these rewritable optical disks.
The structures of these conventional optical disks are next described. Pits and lands/grooves have been previously formed on a substrate of a read-only optical disk by injection molding. A reflective metal layer of an Al alloy is formed on the pits and lands/grooves.
In the WORM optical disk, an alloy material of Te, Bi, Se, Sn, or other low-melting-point material is formed as a thin layer or a dye material is applied as a thin layer on a substrate. The dye material shows absorbing characteristics at the wavelength of the used laser light. This fabrication step results in a recording layer.
A rewritable MO disk uses a sandwich structure having a thin film of an alloy of Tb, Gd, Dy, Ho, or other rare-earth metal and Fe, Co, Ni, or other transition metal. This thin film is sandwiched between transparent protective films of SiN or the like.
In a rewritable, phase-change optical disk, a thin film of chalcogenide-based material such as GeSbTe, AgInSbTe or InSbTe is used as a recording film. Examples of material of thin films acting as recording layers of phase-change optical disks include InSe, InTe, AsTeGe, TeOxxe2x80x94GeSn, TeSeSn, SbSeBi, and BiSeGe. These thin films are formed by resistive heating vacuum evaporation, electron-beam evaporation, sputtering, or spin-coating techniques.
Although there is a demand for larger capacities and higher densities in this way, the conventional read-only, write-once-read-many, and rewritable optical disks use a single-layer structure. That is, formed on a substrate having transparency and rigidity is a write-once-read-many (WORM), MO, or phase-change recording film, as well as pits and lands/grooves.
Furthermore, a partial ROM disk fabricated by forming a read-only portion, a write-once-read-many (WORM) portion, and a rewritable portion that are separated from each other in a radial direction of a substrate has been proposed and manufactured (for example, see xe2x80x9cErasable Optical Disk Techniquesxe2x80x9d, p. 249, Triceps Publishing Company, Japan, 1991).
However, the aforementioned optical disk and hybrid disks are of the single-layer structure and so it is difficult to increase their recording capacities drastically. In the case of a double-layer disk fabricated by gluing together disks to improve the recording capacity, the head structure must be modified to permit simultaneous access to both layers. This increases the cost. Hence, it is difficult to implement this scheme.
Accordingly, with respect to read-only optical disks, read-only multilayer disks that permit access from one side to two or more layers has been proposed (e.g., Proc. of The International Society for Optical Engineering, Vol 2338, xe2x80x9cMultilevel Volumetric Optical Storagexe2x80x9d, by K. Rubin et al., WA3-1, Optical Data Storage Meeting Technical Digest (1994). In addition, a method of reading information from two layers on one side, based on read-only DVD standards that stipulate two-layer disks, has been adopted.
Where improvement of the recording density of optical disks is considered, rewritable optical disks permitting rewriting of information are quite useful. Therefore, it has been required to secure practical performance with one-side multilayer structure, and this technique has been discussed.
Accordingly, an optical information recording medium having two recording layers as a rewritable optical disk utilizing phase changes has been proposed (Japanese Patent Laid-Open No. 157830/1991). Additionally, two-layer recording on a phase-change optical disk has been reported (for example, an Extended Abstract of The Japan Society of Applied Physics, titled xe2x80x9cDual Layer Type Rewritable Phase Change Optical Diskxe2x80x9d by Nagata et al., 15a-V-5, p. 1008, the 59th Autumn Meeting, 1998, Sep. 15, 1998).
With the known optical information recording medium described in the above-cited Japanese Patent Laid-Open No. 157830/1991, recordings are not made on two recording layers. Rather, a two-layered structure is merely adopted as a means for applying a desired phase difference to reproduced signals. In consequence, improvement of the recording capacity is not sufficient.
In the case of the optical disk reported in the Applied Physics Society of Japan described above, an optical change in the first recording layer normally makes a great difference in transmittance between a recorded portion and an adjacent non-recorded portion. Therefore, the amount of light transmitted to the second layer differs greatly, depending on whether a recording is made on the first layer or not. For this reason, good recording and reading cannot be accomplished unless the power level of light shone during recording and reading on the second layer is changed.
In view of the foregoing problems, the present invention has been made.
It is an object of the present invention to provide an optical information recording medium capable of making substantially uniform the transmittance of a phase-change recording film, irrespective of whether each location contains a bit of information, preferably permitting good recording, reading, and erasing of information at a high density.
It is another object of the invention to provide a method of recording, reading, and erasing information well at a high density on the optical information recording medium described just above.
An optical information recording medium in accordance with the present invention has a transparent substrate and phase-change optical recording layers laminated on the substrate. Each phase-change optical recording layer has a phase-change recording film, a transparent upper protective film formed on top of the phase-change recording film, a transparent lower protective film formed under the phase-change recording film, and a foundation interference film formed under the transparent lower protective film. Information is recorded and erased on the phase-change recording film by a phase change caused by illumination of laser light. The transparent upper protective film protects the phase-change recording film against thermal deformation during recording and erasing of information. The transparent lower protective film protects the phase-change recording film against thermal deformation during recording and erasing of information. The foundation interference film adjusts the transmittance of the phase-change recording film for the laser light.
In the present invention, the foundation interference film is formed to adjust the transmittance of the phase-change optical recording layer for laser light. Therefore, the transmittance is kept almost constant irrespective of the presence or absence of recorded information. Accordingly, high-density, good recording, reading, and erasing of information are made possible by stacking such phase-change optical recording layers.
Preferably, the transmittance of recorded regions of the second phase-change recording film is 0.90 to 1.10 times the transmittance of unrecorded regions.
The aforementioned phase-change recording film contains at least one element selected from the group consisting of Ge, Sb, and Te.
The first transparent upper protective film, first transparent lower protective film, second transparent upper protective film, and second transparent lower protective film can contain ZnS and SiO2.
The above-described foundation interference film contains at least one element selected from the group consisting of Si and Ge.
The transparent films described above may be made of a radiation-curable resin.
The above-described optical information recording medium further includes a light transmitting spacer formed on the phase-change optical recording layer and a second phase-change optical recording layer formed on the, light transmitting spacer. The second phase-change optical recording layer has a second phase-change recording film on which information is read and erased by a phase change caused by illumination of laser light, a second transparent upper protective film formed on top of the second phase-change recording film, and a second transparent lower protective film formed under the second phase-change recording film. The second transparent upper protective film protects the second phase-change recording film against thermal deformation during recording and erasing of information. The second transparent lower protective film protects the second phase-change recording film against thermal deformation during recording and erasing of information.
Two optical information recording media, each having the construction described above, may be bonded together by bonding together their top phase-change optical recording layers.
A method of recording, reading, and erasing information in accordance with the present invention is for use with an optical recording medium having a light transmitting substrate and plural phase-change optical recording layers laminated over each other via light transmitting spacers. The top layer of the phase-change optical recording layers has a first phase-change recording film for recording, reading, and erasing information by a phase change caused by illumination of laser light, a first transparent upper protective film formed on top of the first phase-change recording film, and a first transparent lower protective film formed under the first phase-change recording film. The first transparent upper protective film acts to protect the first phase-change recording film against thermal deformation during recording and erasing of information. The first transparent lower protective film serves to protect the first phase-change recording film against thermal deformation during recording and erasing of information. Each of the other phase-change optical recording layers includes a second phase-change recording film for recording and erasing information by a phase change caused by illumination of laser light, a second transparent upper protective film formed on top of the second phase-change recording film, a second transparent lower protective film formed under the second transparent recording lower protective film, and a foundation interference film formed under the second transparent lower protective film. The second transparent upper protective film protects the second phase-change recording film against thermal deformation during recording and erasing of information. The second transparent recording lower protective film protects the second phase-change recording film against thermal deformation during recording and erasing of information. The foundation interference film adjusts the transmittance of the second phase-change recording film for the laser light. This method consists of directing laser light to the optical information recording medium from a side of the light transmitting substrate while varying the focal point to cause a phase change in at least one of the first and second phase-change recording films.